Catalytic device for the implementation of a reaction in a gaseous medium at high temperature

ABSTRACT

The present invention relates to a catalytic device for the implementation of a reaction in a gaseous medium at high temperature, such as, for example, the synthesis of HCN or the oxidation of ammonia, comprising: at least one textured material ( 1 ) which is effective as catalyst for the said reaction, a support ( 2 ) comprising at least one ceramic part ( 3 ), the structure of which makes possible the passage of the gases, the said part ( 3 ) of the said support ( 2 ) having a corrugated face ( 6 ), so that the increase in surface area (β) produced by the corrugations with respect to a flat surface is at least equal to that (α) calculated for sawtooth corrugations and of between approximately 1.1 and approximately 3, the said textured material ( 1 ) being positioned so that it is held against the corrugated face ( 6 ) of the said part ( 3 ) of the said support ( 2 ) and follows the form thereof.

The present patent application is a non-provisional application ofInternational Application No. PCT/IB01/01692, filed Jul. 27, 2001.

The present invention relates to catalytic reactions in a gaseous mediumat high temperature, for example the oxidation of ammonia and thesynthesis of HCN. A particular subject-matter of the invention is animproved catalytic device which can be used in this type of reaction anda reactor comprising it.

The oxidation of ammonia is widely used in the production of nitricacid. The process, known as the Ostwald process, comprises the stage ofpassing a preheated ammonia/air mixture, typically comprising 5–15%, inparticular 10–12% of air by volume, with a high linear velocity(measured under standard temperature and pressure conditions) through acatalytic device extending over the transverse cross-section of thereactor.

The synthesis in a single operation of hydrocyanic acid (HCN) fromammonia and a gaseous hydrocarbon, in which synthesis the heat neededfor the endothermic reaction is provided by simultaneous combustionreactions with oxygen or a gas comprising oxygen, such as air, in thepresence of a catalyst, is an operation which has been known for a greatmany years (U.S. Pat. No. 1,934,838). It is known as the Andrussowprocess.

These two types of reaction use catalysts from the platinum group,generally in the form of a flat woven gauze. The apparent workingcross-section of these catalysts is limited by the dimensions of thereactor.

In order to increase the productivity of these reactors, it is possibleto increase the number of catalytic gauzes. However, beyond a certainthickness, the pressure drop thus created opposes the increase in theflow of reactant and nullifies the effects of a better conversion yield.In addition, the increase in thickness can promote side reactions. Thus,the difficulties in increasing production in the current state of theart are due:

-   -   to the pressure drops,    -   to the number of active sites of the catalyst (contact surface        area),    -   to the contact time of the catalyst with the reactants.

With the aim of increasing the effective surface area of the catalyst,U.S. Pat. Nos. 5,160,722 and 5,356,603 provide for the use of catalystgauzes having transverse corrugations. Although this surface area isthus increased, these corrugations have low amplitudes. Furthermore, theshape-retention of this assembly is only really possible fortemperatures of less than 800° C. Beyond this, the mechanicalcharacteristics of the metal become insufficient and, from the increasein the pressure drop, the folds or corrugations have a tendency to sag.The lifetime of such a device is thus very short and thus incompatiblewith industrial operation.

Patent Application EP 931,585 for its part discloses the use of acatalyst gauze in the form of radially corrugated discs or cones, sothat a revolving burner can follow the corrugations as it rotates aboutits axis. However, the abovementioned problems remain.

An aim of the present invention is therefore to provide a catalyticdevice comprising a catalyst which has a greater geometric surface areaand which withstands the reaction conditions without substantiallyincreasing the pressure drop or the side reactions.

The present invention relates to a catalytic device for theimplementation of a reaction in a gaseous medium at high temperature,such as, for example, the synthesis of HCN or the oxidation of ammonia,comprising:

-   -   at least one textured material which is effective as catalyst        for the said reaction,    -   a support comprising at least one ceramic part, the structure of        which makes possible the passage of the gases, the said part of        the said support having a corrugated face, so that the increase        in surface area (β) produced by the corrugations with respect to        a flat surface is at least equal to that (α) calculated for        sawtooth corrugations and of between approximately 1.1 and        approximately 3,        the said textured material being positioned so that it is held        against the corrugated face of the said part of the said support        and follows the form thereof.

The means which makes it possible to hold the textured material againstthe corrugated face of the part of the support is advantageouslycomposed of a second part of the ceramic support, the structure of whichmakes possible the passage of the gases,

the said second part having a corrugated face which is substantiallyhomologous and complementary with the said corrugated face of the saidfirst part of the support

and the said second part being positioned so that the said corrugatedfaces of the said first and second parts are facing and that thetextured material is inserted between the said corrugated faces andfollows the form thereof. Thus, the pressure drop is advantageouslysubstantially homogeneous over the whole of the catalytic device thusformed.

Other conventional means known to a person skilled in the art can beused to hold the textured material against the corrugated face of thefirst part of the support.

The term “textured material” is understood to mean, within the sense ofthe present invention, any assemblage of strips or wires which arelinear and/or in the form of helical components which makes possible thepassage of the gases. This assemblage is, for example, of the gauze,woven fabric, knitted fabric or felt type and can be obtained by varioustechniques, such as weaving, knitting, sewing, embroidery, and the like.It is advantageously a gauze.

The term “two corrugated faces of substantially homologous andcomplementary form” is understood to mean, within the sense of thepresent invention, any combination of two faces exhibiting corrugationswith a similar size and form, that is to say having a substantiallyidentical increase in surface area β, which is constructed such that,when these two faces are facing, the corrugations are complementary.

Other subject-matters and advantages of the invention will becomeapparent to persons skilled in the art from the detailed descriptionhereinbelow and by means of references to the following illustrativedrawings.

FIG. 1 represents a specific diagrammatic example of a catalytic deviceaccording to the invention.

FIG. 2 represents the parameters which make it possible to calculate theincrease in surface area (α) produced by sawtooth corrugations.

The support (2) according to the invention is made of ceramic, thestructure of which makes possible the passage of the gases. Examples ofthese ceramics are, without limitation, ceramic foams or ceramiccomposites. The support (2) can have a honeycomb structure. The term“ceramic” is advantageously understood to mean, within the sense of thepresent invention, any refractory material capable of withstanding thetemperature to which a catalytic platinum gauze is brought in thereaction medium comprising, inter alia, steam. In the case of theapplication in the synthesis of HCN according to the Andrussow process,this temperature can reach 1200° C. The materials which are suitable aretherefore then based on alumina and can comprise variable proportions ofsilica (10 to 60% by weight) and of magnesium, zirconium, titanium andcerium oxide (1 to 20% by weight for each of these constituents). Thesematerials can comprise, without limitation, one or more of the followingcompounds: silicon dioxide (silica SiO₂), silicon carbide SiC, siliconnitride Si₃N₄, silicon boride, silicon boronitride, aluminium oxide(alumina Al₂O₃), aluminosilicate (3Al₂O₃—2SiO₂), aluminoborosilicate,carbon fibres, zirconium oxide (ZrO₂), yttrium oxide (Y₂O₃), calciumoxide (CaO), magnesium oxide (MgO) and cordierite (MgO—Al₂O₃—SiO₂).

Use will advantageously be made of the ceramic Stetta®G29 from Stettner,the characteristics of which are as follows:

Relative Flexural Linear coefficient Linear Thermal Thermal shockMaximum operating Volume Porosity Density strength of expansioncoefficient conductivity resistance temperature resistance in % inkg/dm³ N/mm² 1/K · 10⁻⁶ at 20− of expansion W/(mK) in ° C. in ° C. at800° C. >3 2 45 1.5–3 2–4 1.3–1.7 380 1000 10⁵Prior to their use, these materials which are used to form the support(2) are generally formed according to known techniques of moulding,extrusion, agglomeration, and the like. They are subsequently calcinedat high temperature (>1300° C.), so as to acquire mechanical propertiescompatible with their future operating conditions. These combinedoperations must confer on them a structure which makes possible thepassage of the gases which can, for example, exist in the form of cellscommunicating in the 3 directions (foams) or of honeycombs with acircular or polygonal (square, rectangular, hexagonal, and the like)cross-section.

The textured material (1) which is effective as catalyst is inparticular a catalytic metal from the platinum group and can be preparedfrom platinum, rhodium, iridium, palladium, osmium, ruthenium or themixture or alloy of two or more of these metals. It is advantageouslyplatinum or a platinum/rhodium alloy. Alternatively, this catalyst canbe a mixture composed of a catalyst from the platinum group as describedhereinabove and at least one other material including but not limited tocerium, cobalt, manganese, magnesium and ceramics.

The corrugations of the faces (6) and (7) of the support can be of anytype, in particular sawtooth corrugations.

The sawtooth corrugations will be defined by the height “h” of eachcorrugation and the distance “d” between 2 corrugations. The increase insurface area α produced by corrugations of this type can thus becalculated from these 2 parameters (FIG. 2) in the following way:α=√{square root over ((4h ² +d ²))}/d

The increase in surface area (β) produced by any type of corrugationaccording to the invention will be at least equal to (α) and will bechosen within the range from approximately 1.1 to approximately 3. Thisis because α=1.1 corresponds to an increase in surface area of 10%.Below this value, the advantages of this corrugation are not veryapparent. Above β=3, the use of such a device becomes difficult. Thesawtooth corrugations according to the invention advantageously have aprofile of an isosceles triangle with d=2h, which results in a ratio αof approximately 1.40 and thus in an increase in surface area ofapproximately 40%.

The present invention also relates to a reactor for an exothermicreaction at high temperature in a gaseous medium having a generallycircular transverse cross-section, characterized in that it comprisesthe catalytic device according to the invention extending across itstransverse cross-section.

It also relates to a reaction process in a gaseous medium at hightemperature, such as the oxidation of ammonia or the synthesis of HCN,characterized in that it uses a catalytic device or a reactor accordingto the invention.

In a specific embodiment of the invention, the process according to theinvention is the synthesis of HCN and comprises the stage of passing agas mixture comprising a hydrocarbon, advantageously methane, ammoniaand oxygen over the catalytic device according to the invention at atemperature of between 800 and 1400° C., so as to obtain, afterreaction, a gas flow comprising at least 5% by volume of HCN.

The hydrocarbon used in the process for the synthesis of HCN accordingto the invention can be a substituted or unsubstituted and aliphatic,cyclic or aromatic hydrocarbon or a mixture thereof. The examples ofthese hydrocarbons include, without limitation, methane, ethylene,ethane, propylene, propane, butane, methanol and toluene. Thehydrocarbon is advantageously methane.

The present invention also relates to a process for the preparation ofthe catalytic device according to the invention, characterized in thatthe textured material (1) is rolled out against the corrugated face (6)of the part (3) of the support (2), so that it follows the form thereof,and in that it is held there with the help of an immobilization means.

This immobilization means is advantageously mechanical and is composedof the second part (4) of the support (2), the corrugated face (7) ofwhich covers the opposite face of the said material (1) to that situatedagainst the corrugated face (6) of the part (3) of the said support (2).

More advantageously still, the combination thus formed generates a lowpressure drop which is substantially homogeneous in the cross-section ofthe reactor.

A specific diagrammatic example of the device according to the invention(FIG. 1) is composed of:

-   -   a combination of corrugated gauzes (1),    -   a corrugated ceramic support (2) in two parts (3) and (4), each        having a corrugated face (6) and (7).

The gauzes (1) are situated between the two faces (6) and (7) of theparts (3) and (4) of the support (2).

The parts (3) and (4) of the support (2) are made of ceramic having ahoneycomb structure with a circular or polygonal (square, rectangular,hexagonal, and the like) cross-section.

Example of the Preparation of the Catalytic Device According to theInvention:

The support (2) according to the invention can be given a corrugatedform either before calcination (during the moulding stage) or aftercalcination, by assembling and adhering together prisms with atriangular cross-section.

A combination of platinum gauzes (1), constituting the catalytic charge,is subsequently inserted between 2 layers (3) and (4) of corrugatedsupport (2). This positioning operation is carried out by rolling out,over the corrugated part (3) of the support (2), a combination of gauzesof elliptical form (1) which will be applied to the corrugations of thepart (3) of the support, the support having been positioned beforehandon the hollow bricks (11) constituting the base of the reactor with acircular cross-section. The width of the elliptical gauzes correspondsto the internal diameter of the reactor. The length is equal to thewidth multiplied by the coefficient β defined previously. The upperlayer (4) of corrugated material makes it possible to immobilize thegauzes (1) mechanically while conferring a homogeneous pressure drop onthe combination over the entire exposed surface. A heat shield (10) issubsequently deposited on the catalytic device thus formed. It makes itpossible to confine the reaction and all the activation energy to thenearest point of the surface of the gauzes (1). The hollow bricks (11)are themselves positioned on the boiler tubes (12) of the reactor, whichare composed of a refractory cement (13).

Performance

The use of the catalytic device according to the invention makes itpossible, for the same reactor, to increase the surface area for contactbetween the catalyst and the reactants. This results, for the sameoverall throughput of reactants, in an improved productivity and aminimal and substantially constant pressure drop, making possibleproduction campaigns of longer duration, all the more so since such adevice withstands the reaction conditions, in particular is notsubjected to mechanical deformation.

The table hereinbelow makes it possible to compare the performance of arepresentative system of the prior art with a catalytic device accordingto the invention.

PRIOR ART: INVENTION: Flat Corrugated system system Coefficient α 1.01.4 Productivity: 2.00 2.16 kg of HCN per tonne of air and kg ofcatalyst Increase in pressure 29 5 drop: % per month of the initialpressure drop Duration of the test: 993 3138 hours of production

1. Catalytic device for the implementation of a reaction in a gaseousmedium at high temperature, comprising: at least one textured material(1) which is effective as a catalyst for said reaction, one supportcomprising one ceramic structure (3), which makes possible the passageof gases, said structure (3) having a corrugated face (6), so that theincrease in surface area (β) produced by the corrugations with respectto a flat surface is between about 1.1 and 3, and at least equal to theincrease of surface area (α) calculated for saw tooth corrugationsaccording to a formula α=√{square root over ((4h²+d²) )}/d in which hrepresents the height of each corrugation and d the distance between twocorrugations, said textured material (1) being positioned so that it isheld against the corrugated face (6) of said structure (3) and followsthe form thereof.
 2. Catalytic device according to claim 1, wherein themeans which makes it possible to hold the material (1) against thecorrugated face (6) of the structure (3) is composed of a second ceramicstructure (4) which makes possible the passage of gases, said structure(4) having a corrugated face (7) which is substantially homologous andcomplementary with said corrugated face (6) of said structure (3) andsaid structure (4) being positioned so that said corrugated faces (6)and (7) are facing and that the material (1) is inserted between saidfaces (6) and (7) and follows the form thereof, the pressure drop thusbeing advantageously substantially homogeneous over the whole of thecatalytic device thus formed.
 3. Catalytic device according to claim 1or claim 2, wherein structure (3) and structure (4) have a honeycombstructure.
 4. Catalytic device according to claim 1, wherein thecorrugations are sawtooth corrugations, so that the increase in surfacearea β=√{square root over ((4h²+d²))}/d in which h represents the heightof each corrugation and d the distance between two corregations. 5.Catalytic device according to claim 4, wherein the increase in surfacearea β is about 1.4.
 6. Catalytic device according to claim 1, whereinthe textured material (1) is a gauze.
 7. Reactor for an exothermicreaction at high temperature in a gaseous medium having a generallycircular transverse cross-section, characterized in that it comprises acatalytic device as defined in claim 1 extending across its transversecross-section.
 8. Reactor according to claim 7, characterized in thatthe exothermic reaction is the synthesis of HCN.
 9. Reactor according toclaim 8, characterized in that the device is placed on the hollow bricks(11) constituting the base of the reactor and is covered with a heatshield.
 10. The catalytic device according to claim 1 wherein thestructure is suitable for the passage of gases suitable for thesynthesis of HCN or the oxidation of ammonia.
 11. A reaction processcomprising: preparing a gaseous mixture; and passing the gaseous mixtureover a catalytic device according to claim 1 or in a reactor accordingto claim
 7. 12. The process according to claim 11, wherein the gaseousmixture comprises organic compounds suitable for the synthesis of HCN.13. The process according to claim 12, wherein the gaseous mixturecomprises a hydrocarbon, ammonia and oxygen and is passed over thecatalytic device according to claim 1 at a temperature of between 800and 1400° C., so as to obtain, after reaction, a gas flow comprising atleast 5% by volume of HCN.
 14. A process for the preparation of acatalytic device according to claim 1 wherein the textured material (1)is positioned against the corrugated face (6) by rolling the texturedmaterial (1) out against the surface (6) of the structure (3), so thatit follows the form thereof.
 15. The process according to claim 14,wherein an immobilization means (4) is positioned against the texturedmaterial (1) such that a surface (7) of the immobilization means (4)covers an opposite surface of the textured material (1) to that situatedagainst the surface (6) of the structure (3).